SSH server gives unhelpful message when XMLRPC server unavailable

[Jonathan Lange]

At the moment, when you start a Launchpad SSH server (say poppy or codehosting) and there's no XML-RPC server available and you try to access that server, you get a very unhelpful, generic authentication failure message.

Instead, the SSH server should send a message to the user about one of our internal services being down, or something similar.

To reproduce the error:
  ./bin/twistd -ny daemons/poppy-sftp.tac
  ssh -p 5022 localhost

(You may have to do some juggling to avoid host key errors).